1. Field of the Invention
The present invention is directed to a method for generating an image using nuclear magnetic resonance signals, and in particular to a method wherein "N/2 ghosts" are substantially eliminated.
2. Description of Prior Art
In general, the generation of an image using nuclear magnetic resonance signals is undertaken by exciting at least a portion of an examination subject with RF pulses while the subject is disposed in a fundamental magnetic field, thereby causing nuclear magnetic resonance signals to arise in the patient. These signals are read out in the presence of first and second gradient magnetic fields, and are sampled in the time domain. The samples acquired in this manner for specific individual pulses of one of the two gradient fields are respectively entered into a row of a measurement matrix, and the measurement matrix is subjected to a Fourier transformation for generating the image.
Many sequences of RF pulses and various gradient magnetic fields are known in the art for obtaining a topically resolved image. The pulse sequence which is required in a given method for the acquisition of a row of the measurement matrix is referred to as a "scan." In some known pulse sequences, the measurement or processing condition for the measured signals changes periodically from scan to scan, i.e. from row to row of the measurement matrix. This occurs, for example, in the echo planar method, as disclosed in German Patent 27 55 956 and in the RARE method as described in the article "Clinical Applications and Methodological Developments of the RARE Technique," Hennig et al., Magnetic Resonance Imaging, Vol. 6, pp. 391-395. Even if only slight deviations occur from line to line, this results in an image artifact known as "N/2 ghosts," which means that in an image matrix of N.times.N points, the actual image appears again within the image field, shifted by N/2 points in the positive and negative directions, with respect to the middle of the image matrix. The shifted image appears with a different intensity than the actual image.